MadSci Network: Development

Re: what are differences beetwin the human eye and the rodents one...

Date: Thu Oct 5 16:58:08 2000
Posted By: Michael Do, Grad student, Neuroscience, Harvard University
Area of science: Development
ID: 962203022.Dv

Dear Dominique,

Your question concerns several tissues that are complex, and also diverse 
in organization among the many species you mentioned.  Therefore, a 
comprehensive answer to your question is beyond the scope of this forum.  
To begin answering it, I will generalize that the eye is constructed so as 
to put an image of the outside world on the retina in an appropriate 
manner, such that any difference in the organization of the retina will be 
tightly correlated with differences in the organization of the rest of the 
eye.  I will thus limit my discussion to divergences in retinal 
organization among mammals.

To talk about differences between retinas, we must first characterize 
a “stereotypical” retina.  As you know, the retina is located in the back 
of the eyeball.  It is analogous to the film in a camera.  That is, the 
retina is the only light-sensitive tissue in the eye.  It is a three 
dimensional array of more than 50 cell types that receive light from the 
world, extract various temporal and spatial properties of that light, and 
transmit the information to the brain.  Light entering the retina will be 
absorbed by photoreceptor cells.  These can either be cones (various cone 
types are more or less sensitive to a given color of light) or rods (of 
which there is only one type, that is very good at sensing low levels of 
light).  These photoreceptors are connected to bipolar cells, which relay 
the information to ganglion cells.  Ganglion cells send their axons out of 
the retina, and the bundle of all these axons is the optic nerve.  This 
nerve connects the retina to deep brain structures that are ultimately 
responsible for visual perception.  This is the vertical pathway: 
photoreceptor to bipolar to ganglion cell.

There are also lateral pathways: horizontal cells connect photoreceptor 
cells together, and amacrine cells mediate connections among bipolar 
cells, among ganglion cells, and among bipolar and ganglion cells.

The pattern of connectivity between a ganglion cell and a pool of 
photoreceptors (that is, which bipolar, horizontal, and amacrine cells lie 
between them) determines the ganglion cell’s activity.  You can get some 
fascinating results from this pattern.  For example, there are ganglion 
cells in your eye that react only when objects move in a certain direction.

This is a lot to take in, so please visit to take a 
look at a picture of the retina.  On the main page, click on the picture 
under "Our research goals."  Most of what I described above is shown there 
both in a real retina and in a helpful schematic.

Having sketched a portrait of the retina, we can talk about a difference 
in its organization among animals.  Eyes are specialized either to focus 
on a point, a line (like the horizon), or some mixture of both.  The 
retina will have a high concentration of cells in an area whose shape 
reflects this specialization.  That is, if the eye is dedicated to looking 
at a point, the retina will have a roughly circular region of high cell 
density called the area centralis.  If the eye is dedicated to looking at 
a line, the retina will have an elongated region of high cell density 
called the visual streak.

Animals that live in habitats with little or no view of the horizon tend 
to have retinas dominated by an area centralis.  In most primates and many 
birds and reptiles, the area centralis is so specialized for high acuity 
vision that the retina is actually thinned there to reduce the scattering 
of light entering the retina (light must go through ganglion and bipolar 
cells before it can be sensed by photoreceptors).  This unique area 
centralis is termed a fovea.  In humans, the fovea takes up 2% of the 
retinal area but accounts for 33% of all ganglion cells.  Thus our vision 
is very sharp, and you can read this from a fair distance away if you look 
straight on (and the screen falls on your fovea).  If you fix your gaze so 
that the screen is just in the corner of your eye, you will find that you 
can barely read this at all. That is because the image is not on your 
fovea, and only a small number of cells are responsible for sensing it.  
Animals that have visual streaks tend to live on the open plain.  This 
organization is ideal for keeping much of the horizon in view, and thus 
avoiding predators.  However, the acuity at any point on the visual streak 
tends to be lower than that of the area centralis.

Now we can try to make connections between this organization on the retina 
level with other aspects of the eye.  For example, animals with an area 
centralis tend to have finer eye movements than animals with a visual 
streak.  If accurate vision is limited to a small area, you can see how 
fine-tuning eye movements so that a moving object can be fixed on that 
small area is advantageous.  Animals with visual streaks do not need eye 
movements so fine, since they can see a relatively large area at 
reasonable acuity by keeping their eyes still.  Thus, the eye musculature 
is likely to be different for animals with these two retina types.  You 
can also imagine how pupils might vary between animals with foveas and 
streaks, admitting light in a characteristic shape.

For more information, I would recommend R.W. Rodieck’s _The First Steps in 
Seeing_ to start.  It is published by Sinauer Associates, copyright 1998.  
For a taste of the staggering diversity of eyes throughout phylogeny, I 
refer you to an article by Russel D. Fernald in Current Opinion in 
Neurobiology, 2000, 10:444-450, titled “Evolution of Eyes.”  I am certain 
that tracing references through these two sources will lead you to most of 
what is known about your topic.


Michael Do

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